The computer algorithm COMPARE provides information regarding the biological mechanism of action of a compound. In this study excellent correlations were obtained for a 2,2'-[(3,3'-dimethoxy[1,1'-biphenyl]- 4,4'-diyl)diimino]bis-benzoic acid (redoxal) and 1-(p-bromophenyl)-2- methyl-1H-naphth[2,3-d]imidazole-4,0-dione (BNID) and two well-studied dihydroorotate dehydrogenase (DHOD) inhibitors, dichloroallyl lawsone (DCL) and brequinar, in terms of antiproliferative activity against tumor cell lines in vitro. When redoxal and BNID were incubated with MOLT-4 cells for 72 hours, 50% growth inhibition was achieved at 0.7 micro and 3.5 micro respectively. After 24 hours incubation, pyrimidine triphosphate pools were shown to be decreased by 50% by redoxal (1 micro) and BNID (0.25 micro). Addition of either uridine (50 micro) or cytidine (100 micro) antagonized the cellular cytotoxicity caused by either drug; uridine corrected the UTP and CTP deficit, whereas cytidine corrected only the CTP deficit. Exposure of MOLT-4 cells to 1 micro of either drug for 18 hours followed by one hour exposure [14C]bicarbonate showed a 97% decrease of incorporation of [14C] into pyrmidine triphosphates accompanied by a 91- and 82-fold increase in radioactive incorporation into L-dihydroorotate (DHO) and N-carbamyl-L-aspartate, respectively. By direct exposure of DHOD prepared from MOLT-4 cell mitochondria to a range of concentration of the two drugs, enzyme IC50 values of 0.21 micro (redoxal) and 0.10 micro (BNID) were achieved at 50 micro DHO. These data provide direct evidence for inhibition of DHOD by redoxal and BNID in MOLT-4 lymphoblasts, and confirm the utility of the COMPARE program in use by DTP, DCT.